1. Field of the Invention
The present invention relates to an apparatus for stabilizing the quantity of light of a fluorescent lamp. The lamp may be employed for illuminating an original picture in a system for duplicating pictures through an optical system by a photoengraving process, and the present invention also relates to a method of stabilizing the quantity of light of a fluorescent lamp.
2. Description of the Prior Art
A fluorescent lamp, which is generally employed as an illumination source, is also applicable in printing processes. Specifically, such a lamp is applicable in color separation processes for producing a color original picture. In such processes, the lamp operates as a cold light source having a relative spectral distribution substantially equal to spectral luminous efficacy and small calorific power. In particular, it is believed that a fluorescent lamp may be used in an image reader employing a recently developed semiconductor optical sensor such as a CCD since a light source such as a halogen lamp (which contains a large quantity of infrared rays as a spectral characteristic) degrades the quality of a duplicated picture image.
Nevertheless fluorescent light sources have not generally been employed in photoengraving processes.
This is because the quantity of light from a fluorescent light source is unstable for a while upon lighting such that the quantity of light fluctuates in a relatively short time. Thus, employment of a fluorescent lamp causes a problem in the field of photoengraving for scanning an original sequentially along lines to read image density information thereof in high density, since errors are caused in read data thereof if the quantity of light for illuminating the original fluctuates during the scanning interval. Therefore, a light source such as a halogen lamp has been employed, because the quantity of light from a halogen lamp fluctuates less.
On the other hand, a copying machine or the like generally requires a short time of about 1 sec. for reading an original (including that of a maximum size, A3: 297mm.times.420 mm), and hence a change in the quantity of light in such a short time can be neglected. Thus, in practice employment of a fluorescent light source causes no problem within a copying machine or the like.
Further, a scanner such as a facsimile also employs a fluorescent lamp as a light source. This is because in a facsimile an image is generally bilevellized in black and white with no intermediate density and a slight change in the quantity of light generally causes no problem.
The quantity of light of a fluorescent lamp is determined by the mercury vapor pressure in the fluorescent lamp and the tube current thereof. The mercury vapor pressure depends on the ambient temperature thereof, which also determines luminous efficiency. In more concrete terms, the lowest point (hereinafter referred to as "coldest point") of the tube wall temperature of the fluorescent lamp determines the mercury vapor pressure as well as the luminous efficiency of the fluorescent lamp. Therefore, the luminous efficiency of the fluorescent lamp can be controlled by providing the coldest point in some portion on the tube wall of the fluorescent lamp and controlling the temperature thereof. On the other hand, the quantity of light of the fluorescent lamp can be stabilized by appropriately controlling its tube current.
FIG. 1 shows an apparatus which has been proposed in the art to stabilize the quantity of light of a fluorescent lamp and distribution thereof. Referring to FIG. 1, light from a fluorescent lamp 1 is received by an optical sensor 2 for monitoring the quantity of light, and output from the optical sensor 2 is inputted in a light quantity feedback unit 4 through an amplifier 3. Output (tube current control signal) from the light quantity feedback unit 4 is supplied to a fluorescent lamp inverter 5, which in turn supplies appropriate tube current to the fluorescent lamp 1 in response to the tube current control signal. The light quantity feedback unit 4 is adapted to control the fluorescent lamp inverter 5 in response to the level of the signal from the optical sensor 2 for adjusting the tube current to be fed to the fluorescent lamp 1, thereby to regularly maintain the output level of the optical sensor 2 at a constant value.
On the other hand, a cooling device 6 such as a Peltier device is brought into contact with a prescribed tube wall portion of the fluorescent lamp 1, in order to control the position and the temperature of the coldest point of the fluorescent lamp 1. A temperature sensor 7 such as a thermister is interposed between the cooling device 6 and the tube wall. The cooling device 6 is controlled by a cooling device driver 8 in response to a value detected by the temperature sensor 7, so that the temperature of the coldest point is maintained at a desired value.
In order to reliably maintain the portion provided with the cooling device 6 as the coldest point, heaters 9 are serially provided in appropriate pitches on the tube wall of the fluorescent lamp 1 except for the portion being in contact with the cooling device 6. A temperature sensor 10 such as a thermister is provided in an appropriate position on the tube wall of the fluorescent lamp 1. The heaters 9 are controlled by temperature control means (not shown) in response to a value detected by the temperature sensor 10, to heat the tube wall of the fluorescent lamp 1, being in contact with the heaters 9, up to a prescribed temperature exceeding that of the coldest point.
In the conventional apparatus as shown in FIG. 1, a desired effect of stabilizing the quantity of light can be attained when the optical sensor 2 receives only the light from the fluorescent lamp 1. If the apparatus is applied to an image scanner, however, an error may be caused since the optical sensor 2 receives light reflected by the surface of an original to be duplicated in addition to the light directly received from the fluorescent lamp 1.
When an original has variable-density gradation, the quantity of light received by the optical sensor 2 is reduced during scanning of a high-density region (dark part) of the original as compared with that during scanning of a low-density region (bright part), whereby the light quantity feedback unit 4 controls the fluorescent lamp inverter 5 to increase the tube current of the fluorescent lamp 1, similarly to the case where the quantity of light of the fluorescent lamp 1 is reduced. During scanning of the low-density region of the original, on the other hand, the light quantity feedback unit 4 controls the fluorescent lamp inverter 5 to reduce the tube current of the fluorescent lamp 1. Therefore, it is impossible to control the fluctuation of the quantity of light of the fluorescent lamp 1 in the accuracy required for scanning of an original in a photoengraving process preferably within about 1%), in the apparatus shown in FIG. 1. Namely, the apparatus shown in FIG. 1 cannot control fluctuation in the quantity thereof within 1%.
A change in density of the original exerts an influence on the quantity of light received by the optical sensor 2 wherever the optical sensor 2 is provided. This presents an inconvenience which cannot be eliminated so far as light quantity feedback control is effectuated during scanning of an original.